1. Field of the Invention
This invention relates to a high resolution scanning optical system used for, for example, laser processing apparatus and the like, and more particularly for a scanning optical system capable of accurately correcting errors in picture-drawing due to irregularity of rotation of a scanning deflector for scanning a drawing beam, and for a scanning optical system capable of accurately detecting the position of a drawing beam deflected by a scanning deflector.
2. Description of the Prior Art
In a scanning optical system such as a laser printer, etc., a polygon mirror is generally used as a scanning deflector. The rotation of a constant angular velocity of a polygon motor used for driving the polygon mirror obtains a constancy of velocity of a drawing light spot on a drawing plane. And thus, the control of the output of drawing data is made easy.
A typical polygon mirror used for this type of use, in general, has a jitter of about 0.03%. However, as an optical system used in the conventional laser printer, etc. has about 1:50.about.1:70 in F number and about 60.about.100 .mu.m in spot diameter, even if a polygon mirror has a jitter of the above-mentioned degree, drawing efficiency of the laser printer, etc., is hardly affected. Further if the drawing light is controlled in accordance with pulse signals from an origin detection light receiving means disposed outside of the effective scanning range, there can be obtained a sufficient drawing quality.
However, drawing laser processing apparatus require such a high degree of accuracy as 1000 dpi (dot per inch) or more in dot density and a spot diameter of a drawing light of an optical system used for it is required to be reduced to about 30 .mu.m.
For this purpose, an optical system of about 1:25.about.1:35 in F number is required. In such a high accuracy apparatus as mentioned, even such minor errors caused by a jitter of the polygon mirror as mentioned above cannot be disregarded, and the jitter must be corrected.
Also, heretofore, a monitor optical system has been used in a scanning optical system. One example of such conventional scanning optical system is illustrated in FIG. 13.
This apparatus, as shown in FIG. 13, is designed such that a beam from a laser 1 is separated into a drawing beam and a monitoring beam by a half mirror 2 and then made incident to a polygon mirror 3 with different angles relative to each other, and both of the beams are simultaneously reflected and deflected by this polygon mirror 3.
The deflected respective beams are focused through an f.theta. lens 4. Among them, the monitoring beam is reflected by a separation mirror 5, then transmitted through a transmission scale 6, then made incident to a introducing means 7 of a scanning position detection light receiving portion, and then reaches a light receiving element 8.
On the other hand, the drawing beam reaches an image surface without passing through the separation mirror 5. In this specification, under the above-mentioned construction, a plane of deflection scanned by a scanning deflector is called a principal scanning plane, and a plane perpendicular to the principal scanning plane and further including the optical axis of the f.theta. lens 4 is called an auxiliary scanning plane. Since the monitoring beam is also scanned through the f.theta. lens 4, a monitoring spot on the transmission scale 6 scans in a constant velocity. If the transmission scale 6, etc. has a constant spatial pitch of transmission sections, it is ideal that an output signal of the same output level can be obtained from the light receiving element 8, and a pulse signal of equal pitches can be obtained from this output signal. Output pulse signals from this light receiving element 8 are used as clock pulses for correctly detecting scanning positions of the image drawing beam.
However, in the above-mentioned monitor optical system, an angle of incidence of the monitoring beam relative to the transmission scale 6 varies between a peripheral portion of the transmission scale 6 and a central portion thereof, except for a special case where the f.theta. lens is telecentric. As a result, the incident efficiency to the introducing means 7 is changed, and the level of the output signal of the light receiving element is not constant.
Also, in case of forming the introducing means 7 by bundling optical fibers 82a (see FIG. 7) whose end portions are faced toward the transmission scale 6, empty spaces are formed among each optical fiber 82a. As the result, the following cases occur sequentially. One case is that a spot of the monitor beam passed through the transmission scale 6 makes incident wholly to the optical fiber's end portion. Another case is that the spot makes incident partially to the optical fiber's end portion and partially to the empty spaces. And in the latter case the incident ratio to the optical fiber's end portion of the spot varies according to its position on the introducing means 7. Then, the detected light quantity varies according to the quantity of incidence to the optical fiber's end portion.
Owing to the above-mentioned causes, peak values of output signals from the light receiving element 8 are changed within one trial of scanning by the monitoring beam on the transmission scale 6 as shown in FIG. 14(a).
If output pulse signals are determined as shown in FIG. 14b, by taking a threshold value at 1/2 of each wave height and with reference to such changing output signals, the threshold values are unstable. Moreover, the pulse widths of FIG. 14(b) become different, and correct output pulse signals become impossible to obtain.